Shell defends against -6 (body) shots and venom. It will wear away slowly as it is hit. The disadvantage of shell is that it makes you heavy and therefore slow. Shell does not defend against -1 shots.
See how much shell you have by referencing .shell
Make shell by storing the amount of shell you want to make (up to 100) into .mkshell. Shell can be removed by setting .mkshell to a negative value.
100 .mkshell store
will make 100 shell